1. Field of the Invention
The invention generally relates to an apparatus and a method for controlling a lock-up clutch of a vehicle, and, more particularly, to a technique for preventing or suppressing occurrence of hunting when the temperature of hydraulic oil supplied to the lock-up clutch is low.
2. Description of Related Art
Generally, a power transmitting system of a vehicle includes a plurality of hydraulic friction devices, and the engagement pressure of each of the hydraulic friction devices is controlled by hydraulic oil supplied from a hydraulic control circuit. However, the hydraulic friction device may be slowly engaged or engaged with a delay when the hydraulic oil has a low temperature before warm-up, for example, since the viscosity of the hydraulic oil is high. In view of this problem, various control apparatuses for automatic transmissions are known which change the hydraulic pressure of the hydraulic oil depending upon the temperature of the hydraulic oil that is associated with the engagement pressure of the hydraulic friction device. For example, a control apparatus for an automatic transmission as disclosed in JP-A-2-42251 is adapted to control the line pressure based on hydraulic oil temperature information supplied from an oil temperature sensor, so as to favorably avoid a delay in the engagement of hydraulic friction devices when the temperature of the hydraulic oil is low.
In a vehicle including a hydraulic torque transfer device equipped with a lock-up clutch, such as a torque converter equipped with a lock-up clutch or a fluid coupling equipped with a lock-up clutch, a control apparatus for the lock-up clutch is conventionally provided for setting a slip region between a release region in which the lock-up clutch is released and an engagement region in which the clutch is fully engaged or applied, and controlling a slip amount to place the lock-up clutch in a half-engaged state in the slip region, in order to improve the fuel economy of the vehicle by further reducing a rotation loss of the lock-up clutch. In the case where a relatively rapid change in the input torque occurs when the lock-up clutch is placed in the half-engaged state (or flexible state), the known control apparatus for the lock-up clutch may not operate with sufficient response if the temperature of the hydraulic oil is low, and thus may suffer from hunting. To solve this problem, the inventor has developed the concept of changing the hydraulic pressure of the hydraulic oil supplied to the lock-up clutch, depending upon the oil temperature of the hydraulic oil.